U.S. Pat. No. 5,166,236 discloses the incorporation of 0.05 (500 ppm) to 5 parts of inorganic phosphor into 100 parts (by weight) of fluoroelastomer composition, but preferably 1 (10,000 ppm) to 2.5 parts of the phosphor, for such purpose as to identify (tag) the fluoroelastomer as to source or production lot history. The highest temperature to which the composition is exposed in preparation is 250.degree. C., but often no more than 190.degree. C., when the composition is cured after fabrication into the desired shape. The fluoroelastomer also contains added colorant (pigment) such as carbon black which colors the fluoroelastomer composition to the naked eye. The phosphor in the fluoroelastomer emits a color which is visible to the naked eye and is distinguishable from the colorant upon exposure of the composition to excitation radiation, i.e., radiation which causes the phosphor to emit it characteristic color.
There is a need for "tagging" non-elastomer fluoropolymers, i.e., fluoroplastics, which are free of added colorant, whereby the purity of the fluoroplastic is visible to the naked eye, and which are exposed to much higher processing temperatures than fluoroelastomers, e.g., at least 300.degree. C. either in the melt fabrication of melt-fabricable fluoroplastics or 350.degree. C. in the sintering of non-melt fabricable fluoroplastics, notably polytetrafluoroethylene. Such a high processing temperature also introduces the problem that even only small amounts of additives introduced into such fluoroplastics typically have the effect of causing the fluoroplastic to degrade during such processing, which can become evident from discoloration of the normal (no additive) appearance of the fluoroplastic. Another problem with using an additive in fluoroplastics, even if visible degradation during high temperature processing of the fluoroplastic does not occur, arises from the use of the fluoroplastic in critical chemical processing environments such as food processing and pharmaceutical and semiconductor manufacture where the fluoroplastic cannot contaminate the food or chemical processing fluid. For this reason, most fluoroplastic is produced "pure", i.e. consisting only of the fluoroplastic. A usual method for determining purity of the fluoroplastic in the field is to subject it to dielectric strength test (ASTM D149), because the dielectric strength of fluoroplastics is very sensitive to the presence of impurities (additives) in the fluoroplastic.